The portal vein is a vessel in the abdominal cavity that drains deoxygenated blood to the liver for cleaning. A system of blood vessels called the hepatic veins remove the cleaned blood from the liver to the inferior vena cava, where it is returned to the heart. Portal hypertension (“PHT”) occurs when the portal vein experiences a rise in blood pressure that is not a consequence of an increase in a patient's overall systemic blood pressure. Often, PHT is defined according to a “portal pressure gradient,” or, the difference in pressure between the portal vein and the hepatic veins, for example of 10 mmHg or greater. A typical portal venous pressure under normal physiological conditions is less than or equal to approximately 10 mmHg, and the hepatic venous pressure gradient (HVPG) is less than approximately 5 mmHg. Increased portal pressure leads to the formation of porto-systemic collaterals; the most serious of them being gastroesophageal varices. Once formed, varices represent a major risk for the patient due to the susceptibility for rupture and subsequent hemorrhage that in many cases leads to death. As a result, PHT is considered the most severe complication of cirrhosis of the liver and is the major cause of morbidity and mortality in cirrhosis patients.
Current procedures for monitoring portal pressure generally involve an indirect measurement of the portal venous pressure through the hepatic venous system. One such procedure is known as the hepatic venous pressure gradient or HVPG. HVPG is used to provide an indirect measurement of the portal vein pressure. The procedure is minimally invasive and involves catheterization of the hepatic venous system via femoral vein or jugular entry. A balloon tipped radiolucent catheter that is capable of measuring local blood pressure usually via a pressure transducer is placed in the Inferior Vena Cava or a large hepatic vein segment. Once in place the pressure is measured to provide the free hepatic venous pressure or FHVP. The FHVP is measured to quantify the external pressures being applied to the venous systems and to zero out the effects of systemic pressure. The catheter is then advanced into a small branch and a complete obstruction of flow is created (wedge position usually done by inflating balloon) to provide the wedged hepatic venous pressure or WHVP. The HVPG is given by HVPG=WHVP−FHVP. While the HVPG has been shown to be a very effective diagnostic and prognostic indicator, it has been limited by the invasiveness of the procedure and the need for standardization to provide reliable results.
Other indirect procedures include, for example, measurement of variceal pressure which employs esophago-gastric approaches to advance an inflatable balloon-catheter into the abdomen of patients via the esophagus and stomach and position the balloon, adjacent to a gastroesophageal varix. The force of inflation required against the wall of the varix is used to calculate the intravariceal blood pressure. In general, non-direct portal venous pressure measurement is less precise, while still invasive and uncomfortable for a patient.
Direct measurement of the portal vein has been attempted in the past. One such procedure involves puncture catheterization, wherein a radiologist accesses the portal and/or hepatic venous systems, under fluoroscopic guidance, by puncturing the tissue of the system with a needle or catheter from outside of the system. Using puncture catheterization, the portal vein may be accessed via a transhepatic puncture using either an intracostal or subxiphoid approach, wherein a needle or catheter is inserted at a patient's 12th vertebrae, between the ribs, and punctures through to the portal vein. The hepatic venous system may be accessed via a transjugular approach, wherein a needle or catheter is inserted into the jugular vein and advanced into the hepatic vein via the vena cava. The portal vein may also be accessed from the hepatic venous system, using an intrahepatic puncture from the hepatic to portal venous systems. Thus, in order to monitor a portal pressure gradient, two separate punctures (for the portal and hepatic veins) are required. Physicians are reluctant to perform frequent, direct portal vein pressure measurements, due to the invasiveness of the procedure and as a result, it is not clinically practiced.
There exists a strong clinical need for a pressure monitoring system that can provide accurate pressure measurements of portal and/or hepatic blood pressure while allowing the physician to monitor those pressures non-invasively.
Conventional devices include active electronics, sensors, and controls which require a power supply, or a connection to the outside world, and which increase the size of conventional devices thus restricting their use in the porto-hepatic venous system. In addition, conventional devices rely on components, for example sensors and/or membranes, that are large and/or needed in plurality of sensors/membranes, in order to maintain functionality, due, in part, to their tendency to rupture.
A need therefore exists for a pressure measurement system that is small in size, sensitive in function, and does not require redundancy. In addition, a need exists for a sensor system that may be operated without the need for wires or cables to transmit the pressure experienced by the sensor to an external device. The pressure measurement system should be miniature, passive, implantable and wireless to allow for non-invasive, frequent monitoring of portal venous pressure.